Display device

The present application claims priority over Japanese Application JP 2008-046233 filed on Feb. 27, 2008, the contents of which are hereby incorporated into this application by reference.

(1) Field of the Invention

The present invention relates to a display device, and in particular, to a technology which is effective when applied to a liquid crystal display device having a direct backlight where LED lights are arranged in a matrix.

(2) Related Art Statement

Some conventional liquid crystal display devices have a direct backlight provided behind the liquid crystal display panel. The above described direct backlight is a backlight where a number of lights are arranged in a region which overlaps with the display region of the above described liquid crystal display panel, and fluorescent lamps or LED lights are used for the above described lights, for example.

Direct backlights using LED lights, as described above, (hereinafter referred to as LED backlights) have low power consumption and can easily be made thin in comparison with direct backlights using fluorescent lamps, as described above. Therefore, more and more liquid crystal display devices have been adopting LED backlights as described above in recent years.

In addition, the above described LED backlights have a number of LED lights arranged in a matrix, as described above, in a region which overlaps with the display region of the liquid crystal display panel, as described above, and thus, the brightness of the above described LED lights is controlled by the applied current. Therefore, the above described liquid crystal display devices having LED backlights have an advantage, such that local control of the brightness, referred to as local dimming or, more simply, dimming, is easy, for example.

The above described local dimming control is a method for dividing the region of the above described LED backlight which overlaps with the above described display region into a number of blocks, so that the brightness in the above described number of blocks can be controlled independently for each block (see for example Patent Document 1). At this time, one or more LED lights are arranged in each of the above described number of blocks in order to provide an LED backlight as that described above. In addition, the brightness of each block is determined on the basis of the average value of the tone of pixels included in the region which overlaps with the block in the display region. Thus, the brightness can be controlled for each block by applying a strong current to LED lights in blocks where the determined brightness is high and applying a weak current to LED lights in blocks where the determined brightness is low.

Thus, the display in regions which overlap with blocks where the brightness of LED lights is low is dark in the display region, while the display in regions which overlap with blocks where the brightness of LED lights is high is bright in the display region. Therefore, in the case where a video or image displayed during a certain frame period includes a portion which locally includes many pixels having a low tone or displaying black, for example, the contrast between this portion and other, bright portions, is higher.

[Patent Document 1] Japanese Unexamined Patent Publication 2007-183608 (Corresponding US Patent application; US2007/152926A1)

The deterioration in the light flux of LED lights depends on the current, so that the light flux deteriorates more rapidly in LED lights to which a strong current keeps being applied than in LED lights to which a weak current keeps being applied.

In the case of local dimming control in the above described LED backlight, the current applied to LED lights is controlled for each block. In addition, the current applied to each LED light is changed during a predetermined period. Therefore, a problem arises with LED backlights where local dimming control is carried out, such that there is a difference in brightness between blocks when the same current is applied to all of the LED lights after long use, for example. This problem easily arises particularly in the case where the same video or images are repeatedly displayed.

An object of the present invention is to provide a technology which makes it possible to delay deterioration in the uniformity of the surface brightness of the above described backlight when all of the lights emit light under the same conditions by using display devices having backlights under local dimming control, for example.

The above described and other objects and new features of the present invention will become clearer from the description in the present specification, as well as the accompanying drawings.

The gist of typical embodiments of the invention disclosed in the present specification is described in the following.

(1) A display device having: a backlight having a number of LED (light emitting diode) lights; a display panel having a display region which is a set of pixels of which the tone is adjusted by changing the transmittance of light from the above described LED lights; a first control means for controlling the tone of each pixel; and a second control means for controlling the brightness of each LED light, wherein the above described number of LED lights are arranged in a region that overlaps with the above described display region of the above described display panel, the display device has an illuminance measuring means for measuring the illuminance in the periphery of the above described display device, and the above described second control means makes the brightness of all of the above described LED lights the same in the case where the illuminance as measured by the above described illuminance measuring means is higher than a preset value, and determines the brightness of each LED light on the basis of the tone of the above described pixels controlled by the above described first control means in the case where the illuminance as measured by the above described illuminance measuring means is lower than a preset value.

(2) The display device according to the above (1), wherein the region of the above described display panel which overlaps with the above described display region is divided into a number of blocks, and one or more LED lights in the above described backlight are provided in each of the above described number of blocks, and the above described second control means determines the brightness of each LED light on the basis of the average value of the tone of the pixels included in the region of the above described display region that overlaps with each block in the case where the illuminance as measured by the above described illuminance measuring means is lower than a preset value.

(3) The display device according to the above (2), wherein the brightness of the above described LED lights is controlled by the current applied to the above described LED lights.

(4) The display device according to the above (3), wherein the above described display device has correspondence data for defining the relationship between the average value for the tone of the above described pixels and the current applied to the above described LED light, and the above described second control means calculates the average value for the tone of the pixels included in the region of the above described display region that overlaps with each block in the case where the illuminance as measured by the above described illuminance measuring means is lower than a preset value, and determines the current to be applied to each LED light on the basis of the above described calculated average value and the above described correspondence data.